fix(cli): preserve dotted agent references

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53243.

Summary

• centralize the CLI @agent mention regex used by import/export
• capture dotted agent names such as @acme.deploy as acme.deploy instead of truncating to acme
• avoid swallowing a sentence-ending . after dotted mentions
• add export/import regression coverage for dotted names

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check
• isolated rustc +1.90.0 --edition=2024 --test harness covering @acme.deploy, @foo.bar., and @my-agent

Targeted cargo +1.90.0 test -p cortex-cli ... is blocked locally on Windows before tests run by the existing aws-lc-sys MSVC C build failure (cl.exe exits while compiling aws-lc). The focused regex harness above validates the changed parsing behavior.

[greptile-apps]

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